This invention relates generally to a method and apparatus for producing proteins from oilseeds and more particularly to a method of producing soy proteins by solubilization and ultrafiltration.
Soy protein products are valued not only as a source of protein, but also as food additives for enhancing texture and other functional characteristics of various products. However, the use of soy protein products is limited due to their beany flavor and tan-like color.
Soy protein has been shown to consist of at least four major fractions. The approximate amounts and molecular weights of each of these fractions, as determined by ultracentrifugation, are set forth in Table 1. The various fractions are identified by sedimentation coefficients. The data in Table 1 was taken from Wolf et al. 99 "Arch. Biochem. Biophys." 265 (1962).
TABLE 1 ______________________________________ Percent Molecular Fraction of Total Component Weight ______________________________________ 2S 22 Trypsin inhibitors 3,000-21,500 Cytochrome c 12,000 7S 37 Hemagglutinins 110,000 Lipoxygenases 102,000 B-Amylase 61,700 7S Globulin 180,000-210,000 11S 31 11S Globulin 350,000 15S 11 -- 600,000 ______________________________________
Researchers have been unable to pinpoint exactly what soybean constituents are responsible for flavor and color, though many compounds are suspected of causing these characteristics. How et al. "Removal of Phenolic Compounds from Soy Protein Extracts Using Activated Carbon," 47 Journal of Food Science 933 (1982) list several organic compounds that are said to produce off-flavors in soy proteins. Among these are aliphatic carbonyls, phenolics, volatile fatty acids and amines, esters and alcohols. Most processes for improving soy protein flavor involve the application of heat, toasting, alcohol extraction or enzyme modification. However these processes normally result in substantial protein denaturation and modification, which alter the product's functionality. In addition, they also promote interaction with lipid and carbohydrate constituents and their decomposition products. Such reactions tend to reduce the utility of soy proteins in most food products, especially in those that require highly soluble and functional proteins, as in dairy foods and beverages.
How et al. used activated carbon to improve flavor and color. However, the activated carbon did not remove the bitter and astringent flavor descriptions and otherwise failed to improve the color above that of commercial soy protein isolates.
Many processes have been described or developed to more efficiently produce soy protein products. Commercial soy isolation processes generally use acid precipitation without resort to ultrafiltration. A typical method of producing soy protein isolates, which are defined as products having at least 90% by weight protein, may include the steps of: (1) extracting the protein from soy flakes with water at an alkaline pH; (2) centrifuging out any solids from the liquid extract; (3) subjecting the liquid extract to isoelectric precipitation by adjusting the pH of the liquid extract to the point of minimum protein solubility to obtain the maximum amount of protein precipitate; (4) centrifuging to separate precipitated protein curd from by-product liquid whey; (5) adjusting the pH of the precipitated protein to slightly below neutral pH; and (6) spray drying the protein to obtain the final product. This process, however, still produces a protein product with a distinctive taste and color.
Commercial soy protein concentrates, which are defined as soy protein products having at least 70% by weight protein, are generally produced by removing soluble sugars, ash and some minor constituents. Sugars are removed by extracting with: (1) aqueous alcohol; (2) dilute aqueous acid; or (3) water, after first insolubilizing the protein with moist heating. McAnelly, U.S. Pat. No. 3,142,571; Sair, U.S. Pat. No. 2,881,076; and Mustakas, G. C. et al. "Flash Desolventizing Defatted Soybean Meals Washed with Aqueous Alcohol to Yield a High-Protein Product," 39 J. Am. Oil Chem. Soc. 222 (1962) are illustrative in this regard. These processes also produce soy protein products with a distinctive taste and color.
Other methods have been developed for producing soy proteins, which employ different types of protein extraction. For example, Frazeur, U.S. Pat. No. 3,728,327 relates to a method for obtaining soy protein relying on homogenization to obtain a fine dispersion which is subjected to some centrifugal separation. The liquid extract obtained from the centrifugal separation is then subjected to reverse osmosis. The retentate from the reverse osmosis is dried to produce the final product.
A number of processes make some use of ultrafiltration in producing soy or other protein products. For example, Goodnight, U.S. Pat. No. 3,995,071 relates to a process for preparing a soy protein having a greatly reduced phytic acid content by the steps of aqueous extraction of defatted soy flakes, basification to a pH in excess of 10.1 and the removal of insolubles. More particularly, the process is carried out by the steps of: (1) extracting a soy protein in an alkaline solution and removing the sediment by centrifugation; (2) adding a base to the extract to raise the pH to a level of 10.1 or greater; (3) separating the resultant solids by centrifugation; (4) neutralizing the extract; and (5) subjecting the extract to ultrafiltration to retain the protein while allowing the lower molecular weight compounds to pass. A semipermeable membrane capable of retaining protein, having a minimum molecular weight in the range of about 10,000-50,000 daltons may be used. Retentate is preferably maintained at a temperature of about 45.degree. C. during the ultrafiltration process and a diafiltration or washing operation may be used on the retentate to eliminate any remaining low molecular weight constituents.
Iacobucci, U.S. Pat. No. 3,736,147 discloses a process for preparing soy and other protein products having a low phytic acid content. Ultrafiltration is preferably carried out in a membrane having a molecular weight cutoff of 10,000-30,000 daltons with the lower limit preferred.
Other exemplary processes are described in U.S. Pat. Nos. 4,088,795; 4,091,120 and 4,072,670, all of which are by Goodnight et al. U.S. Pat. No. 4,072,670 is directed to a method for preparation of a purified soy protein having a lower phytic acid content, while U.S. Pat. No. 4,091,120, employs an ultrafiltration membrane to separate a portion of soybean carbohydrates and mineral constituents to produce a soy protein solution from an extracted aqueous solution of soy protein.
U.S. Pat. No. 4,088,795 is directed to a membrane filtration process for eliminating soluble carbohydrates from an aqueous oilseed lipid-containing suspension. Soybean is preferred as the oilseed. The ultrafiltration membrane has a molecular weight cutoff of about 10,000 to 50,000. The solubilized protein feed is concentrated by the ultrafiltration of the feed to give a volume of permeate equal to one-half the volume of the feed. Removal of additional carbohydrates and mineral constituents is then accomplished by continuously adding a diafiltration solution such as water to the retentate as it circulates through the filtration system in order to wash out the carbohydrates and mineral constituents.
Other processes have been developed using some form of ultrafiltration membranes to produce protein products from other than soybean. For example, O'Conner, U.S. Pat. No. 3,622,556 discloses a method for isolating protein from sunflower meal. Pursuant to one embodiment of the disclosure, the sunflower meal is first subjected to a conventional alkali extraction step under an inert gas blanket. The extract, which contains water, protein, green color forming precursors and other constituents, is then subjected to ultrafiltration under an inert gas blanket. In an alternate method the retentate is next subjected to further ultrafiltration. The inert gas blanket is required to prevent formation of the color causing compound. The ultrafiltration membrane should have a pore size sufficient to allow molecules having a molecular weight of 100,000 to 10,000 and less to pass through while not permitting passage of higher molecular weight molecules such as proteins.
Maubois, U.S. Pat. No. 3,993,636 relates to a process for obtaining protein isolates from sunflower and colza without resorting to an inert atmosphere. Pursuant to that process, ground seeds are first dissolved in an alkaline solution. The solution is then ultrafiltered at a temperature of 2.degree.-30.degree. C. to obtain a retentate having an amount of nitrogenous matter in the range of 3-12% by weight. The retentate is then treated by several more steps, including: continuing ultrafiltration at the same temperature but with a volume of wash liquid, adjusting the temperature of the retentate to 20.degree.-60.degree. C. while continuing the ultrafiltration procedure and thereafter recovering concentrated retentate. The membranes used in ultrafiltration have a well defined cutoff in the range of 2,000-30,000.
The inventor of the present inventive apparatus and method has also developed a number of processes for producing protein products. For example, Lawhon et al., Alternate Processes for Use in Soy Protein Isolation by Industrial Ultrafiltration Membranes, 44 Journal of Food Sciences 213 (1979) discusses the use of calcium hydroxide to extract a soy protein followed by ultrafiltration and spray drying of a protein isolate. The molecular weight cut-off of the membranes for ultrafiltration is set in the range of 10,000 to 18,000 with a goal toward obtaining as much protein as possible without sugars and ash.
These and other processes suffer from one or more of several limitations or disadvantages including reduced functional characteristics in the resulting protein product and the production of a product which has both a distinct or "beany" flavor and an off-color such as a dark cream to light tan color.
These and other disadvantages or limitations are substantially minimized, if not eliminated, by the present invention.